Method of oil recovery by in situ combustion



United States Patent METHGD OF OIL RECGVERY BY IN SITU CONEUSTION Clarke N. Simm, Fullerton, Calii, assignor to California Research Corporation, San Francisco, alif., a corporation of Delaware No Drawing. Application September 11, 1953, Serial No. 379,729

3 Claims. (Cl. 166-11) This invention relates in general to improvements in the production of gas and oil from subterranean petroliferous deposits and relates in particular to an improved method for the recovery of such gas and oil by in situ combustion of a portion of such deposits.

It is well known that conventional or primary methods of recovering gas and oil from subterranean formations, such as by pumping, often do not result in recovery of more than 40 per cent of the gas and oil present in the formation. Numerous so-called secondary recovery methods, such as water flooding and gas drive by repressuring of the reservoir are being practiced, but even these methods have not resulted in as complete and economical a recovery of the available gas and oil as desirable. Also, numerous methods have been proposed for improving recovery by thermal means involving in situ combustion of a portion of the deposit either to lower the viscosity of the oil to facilitate production thereof or to crack and distill the oil to obtain the more volatile portions thereof. In some of these methods it has, been proposed to heat a portion of a formation to establish a combustion zone therein and to move this combustion zone through the formation by the introduction into the formation of free oxygen to support the combustion.

However, insofar as I am aware, it has not been possible by the teachings of the prior art to maintain the combustion zone continuously operative, or to control its progress through a petroliferous formation with sufiicient certainty and accuracy to achieve an optimum condition of recovery of the petroliferous deposits. This invention pertains to the latter type of recovery, and particularly to a method for maintaining stable combustion and accurately controlling the progress of a combustion zone through a petroliferous formation.

I have discovered that the progress of the combustion zone may be controlled by varying the mass velocity of the free oxygen delivered to the combustion zone in accordance with certain characteristics of the petroliferous formation being operated upon, and I have found that there exists a well-defined range within which the mass velocity of the free oxygen must be maintained to maintain a thermally stable combustion zone and to provide efficient recovery from a given formation.

The mass velocity of a fluid is usually stated in units of pound-moles of fluid per unit time per unit of crosssectional area of the medium with respect to which the velocity is measured. This critical range of the mass velocity of free oxygen for underground combustion purposes is dependent upon both the oil saturation and the porosity of the formation in which the combustion is taking place. The lower limit of the free oxygen mass velocity is determined by the requirement for sufiicient oxygen to maintain combustion. The upper limit of the free oxygen mass velocity is determined by the necessity for maintaining the temperature of the combustion zone below the value at which fusing and sintering of the formation occurs, to avoid excessive loss of thermal energy to such fusion and sintering.

Patented Nov. 27, 1956 It is therefore an object of this invention to provide an improved method for recovery of gas and oil from subterranean petroliferous deposits utilizing in situ combustion of a portion of such deposits.

It is an additional object of the present invention to provide an improved method of recovering gas and oil from subterranean petroliferous deposits by means of in situ combustion of a portion of such deposits in which the mass velocity of the free oxygen introduced to support such combustion is maintained within predetermined limits.

It is a further object of this invention to provide a method of recovering gas and oil from subterranean petroliferous deposits by means of in situ combustion of a portion of such deposits in which the mass velocity of the free oxygen introduced to support such combustion is maintained within predetermined limits in dependence upon the porosity and the oil saturation of the formation in which such deposits occur.

Objects and advantages other than those described above will be readily apparent from the following dee scription.

Briefly, the present invention embodies the steps of heating a portion of the formation containing the do: posits to initiate combustion in the deposits, supplying free oxygen to support the combustion and to cause the combustion zone to advance through the formation, maintaining the mass velocity of the free oxygen thus in: troduced within predetermined limits to control the com.- bustion zone, and recovering gas and oil from a suitable production outlet or outlets spaced from the point of origin of the combustion.

The formation is preferably penetrated by at least one input well through which combustion is started and through which the free oxygen is introduced. Combustion may be initiated in the formation by placing in the input well adjacent the formation any suitable source of sufiicient thermal energy, such as an electric heater, 9. gas-air burner or substances capable of producing an exothermic chemical reaction. The formation is. thus heated to initiate combustion in the portion of the petroliferous deposit surrounding the input well bore. The temperature at which combustion begins will vary in dependence upon the nature of the deposits, but I have found that it is necessary to establish a temperature of at least 700 F. in the burning deposit to establish a stable combustion zone.

After initiation of combustion in the deposit, a gas containing free oxygen may be introduced into the forma tion to support the combustion and to cause the cornbustion zone to move out into the formation. As the combustion zone advances out into the formation, there exists in the formation five more or less distinct zones in which different phases of the operation are occurring. These zones may be termed the preheating zone, the com: bustion zone, the cracking zone, the distillation zone and the condensation zone. These different zones are adjacent each other and are listed in the order of their oecurrence proceeding from the input well bore.

The preheating zone comprises the portion of the formation between the input well and the combustion zone and through which the combustion zone haspassed. This preheating zone is thus at an elevated temperature and the residual heat of the reservoir rock in this zone serves to raise the temperature of the incoming oxygen-bearing gas. The preheated incoming oxygen-bearing gas from the preheating zone then enters the combustion zone where it combines with the carbonaceousresidue to produce combustion. The hot gases from this combustion are forced ahead of the combustion zone where they crack the heavy oils in the cracking-zone. Thiscracking produces light hydrocarbons which are carried on out of thecracking zone by the gas flow, and also produces the carbonaceous residue or material which remains in place to serve as fuel when the combustion zone reaches that location. 7

" The partially cooled combustion gas, after leaving the cracking zone, enters the distillation zone where this gas serves to volatilize the light and intermediate hydrocarbons present and transport them downstream as vapor. As the hot combustion gas is further cooled by heat transfer to the formation rock, the volatilized hydrocarbons are condensed in the condensation zone and these con- 'densed products are transported toward the production outlet or outlets by the pressure of the gases produced in the combustion zone.

The effectiveness of the process is dependent upon the fact that only a relatively narrow band or volume of the formation is elevated to the temperature of combustion at any one time. The thermal requirements within this small portion consist of the heat required to heat the incoming oxygen-bearing gas to the temperature of the formation and the heat required to replace that lost by conduction beyond the boundaries of the oil-bearing formation. The heat capacity of the incoming gas is low 'and the thermal conductivity of the formation is also low, so that only a small percentage of the total petroliferous deposit need be burned to furnish the required thermal energy.

As stated above, I have found that there exists a definite range within which the mass velocity of the free oxygen must be maintained to provide a continuously stable and eflicient burning operation. This range, as has been noted, is dependent upon the porosity and the oil saturation of the particular formation being burned, because the product of these two quantities is a measure of the amount of oil present in the formation and is thus a measure of the thermal energy required to remove the oil from the deposit. I have found that a stable condition of combustion can be maintained without applying additional heat from other sources, and the rate of advance through the formation of the zone of combustion can be controlled, by controlling the mass velocity of oxygen in a manner to be described hereinafter. The

lower limit of mass velocity of free oxygen required to is the porosity of the formation expressed as a decimal,

and

S is the oil saturation of the formation expressed as a I decimal.

The upper limit of the mass velocity of free oxygen may be expressed by the equation V=l.08 S pound moles/square foot/hour (2) where V, o and S again represent mass velocity, porosity and oil saturation, respectively. I

Equations 1 and 2 are thus measures of the minimum and maximum masses of oxygen which must be made available to the combustion zone per unit time per unit cross-sectional area of the combustion zone to maintain thermally stable and efiicient operation.

, I have found that if the mass velocity of free oxygen falls substantially below the value set forth in Equation 1, combustion becomes thermally unstable and is extinguished for lack of oxygen unless heat is suppliedfrom a source otherthan the combustion. Similarly, if the mass velocity of free oxygen substantially exceeds the value expressed by Equation 2, the temperature of the combustion zone rises to-a point where fusing and sintering of the formation occurs, resulting in a loss of thermal energy from such fusing and sintering with a consequent inefliciency of operation. The critical limits of the mass velocity of free oxygen are independent of the water saturation of the formation up to water saturations in the range of percent and above, at which values of water saturation the heat required to vaporize this water is excessive.

For a given formation, the values of 1 and S in Equations 1 and 2' will be substantially constant, so that the equations can be stated directly in terms of pound moles of oxygen per square foot of cross-sectional area of the burning portion of the formation per hour for a given formation. For example, for a formation having an average porosity of 30 percent and an average oil saturation of 40 percent, Equation 1 for the minimum mass velocity of free oxygen can be written as V=(0.016) (.3) (.4) pound moles/ftF/hr.

=0.00019 pound moles/ftP/hr.

Similarly, for this formation, Equation 2 for the maximum mass velocity of free oxygen may be written as V=(1.08) (.3) (.4) pound moles/ft. /hr. =0.013 pound moles/ftF/hr.

It will be noted that Equations 1 and 2 set forth the limits of mass velocity of free oxygen in terms of unit cross-sectional area of the burning portion of the formation. This means that the total free oxygen introduced to support combustion must be progressively varied as.

the total cross-sectional area of the burning formation varies with progression of the combustion zone away from the input Well bore to maintain the mass velocity per unit area within the limits of Equations 1 and 2. The mannerin which the ,total burning area varies with progression of the combustion zone is a function of the spacing of the outlet or outlets with respect to the input well. In the case of radial flow, the combustion zone will progress radially outwardly from an input well. Assuming that the oil-bearing formation is overlain and underlain by relatively impervious formations, the combustion zone will be confined to the oil-bearing formation and will be in the form of a cylinder having, a radius dependent upon the distance the combustion zone has progressed from the input well. If R represents the distance which the combustion zone has progressed from the input well and T represents the vertical thickness of the oil-bearing formation, then the cross-sectional area A of the combustion zone at any time is equal to .1 v A=21rRT The manner in which the total cross-sectional area of the combustion zone varies with the advance of the combustion zone may be similarly computed for various other input and output well spacing patterns and these variations may be used to determine the maximum and minimum limits of the total free oxygen required per hour to maintain stable and efiicient combustion. However, regardless of the particular well spacing patterns utilized and regardless of the manner in which the total crosssectional area of the combustion zone varies with advance of the combustion zone, Equations 1 and 2 may be used to determine the limits of the mass velocity of free oxygen per square foot of cross-sectional, area of the buming formation per hour. Equations 1 and 2 are therefore applicable toany type of well spacing pattern.

For a given input and producing well spacing pattern, the :rate at which .the combustion zone advances through the :formationwill'determine the rate at' which'the crosssectional area of'the combustion zone vafies. "The rate of advance'of the combustion zone is a direct function of 'both the amount of free oxygen supplied "to the combustion zone and the concentration of 'amount per-unit volume of oil in the formation. This 'rate of advance of the combustion zone maybe rea'dily calculated to 'determine the'rate at which thetotal cross sectionalarea of the combustion zonevaries with advance of the combustion zone. Thistrate'of chan'geof total cross-sectional area may then be utilized-as an -ai'd in determining the total amount of free oxygen required to maintain the mass velocityof the oxygen' with-in thelimits of Equations 1 and 2.

For example, in the case of the radial flow system described above, assume *that the combustion zone has progressed a distance of fifty feet from the input well. If the formation has a vertical thickness of 20 feet, the cross-sectional area of the combustion zone at this instant Assuming that air is being supplied to the input well at the rate of 25,000 ft. per hour at 60 F. and that the air 'has an oxygen content of 20 percent, the number of pound moles of free oxygen being supplied per hour is and the number of pound moles of free oxygen per hour per unit cross-sectional area of the .combustion Zone is 13.2 2 -00021 pound moles per ft. per hour Assuming that the formation has a porosity of 20 percent and an oil saturation of 50 percent, the number of barrels of oil per cubic foot of formation is (.2)(.5) 3 5.615 -0.0l7 8 barrels/ft.

It has been found that the amount of combustion air required to recover one barrel of oil by this method is 10,000 cubic feet and that the amount of oil recovered averages 90 percent of the total oil in the formation. Therefore, the pound moles of free oxygen required to move the combustion zone through one cubic foot of formation is (0.0178) (0.9) (10,000) (0.2) 379 =0.0845 pound moles per cubic foot Therefore, the rate of advance of the combustion front at a radius of fifty feet is 0.0021 pound moles per ft. per hour 0.0845 pound moles per ft.

=0.025 feet per hour The free oxygen may be either continuously or intermittently injected into the formation to maintain the mass velocity within the limits of Equations 1 and 2. In some cases it may be desirable to introduce the free oxygen intermittently and at a mass velocity approaching the upper limit of Equation 2 so that the average mass velocity over a period of time falls well within the limits defined by Equations 1 and 2.

The mass velocity of the free oxygen is a function of both the free oxygen content of the injected combustion gas and of the rate at which such gas is injected. The mass velocity may accordingly be maintained within the limits defined by Equations 1 and 2 by varying the flow rate of the injected gas or by varying the free oxygen content of the injected gas by suitable means such as the use of suitable diluent gases, or by variations of both of these quantities.

Combustion in situ was accomplished in a number of natural formations under conditions which permited various characteristics ofthe combustion phenomenonre'stilting from the application of this invention to be 'determined. Each of these natural formations was penetrated by a two inch pipe for thcinjection of the oxygen bearing gas. Combustion was started in'each of the formations with a gas-air burner capableof supplying 400 B. t. -u.s per minute. The burner'was disposed'inside'the injection pipe, and the burner exhaust was concentrated on the spherical surface created at the bottom of'the injection pipe. After a few minutes of priming with the 'g'as-a'ir burner to obtain combustion-in the oil around the lower end of the injection pipe, air was injected downthe "annulus between the injection pipe and the burner tubing.

"I'he'air injection rate ranged from Oto I00 cubic feet per minute, in dependence upon the desired'average rate of advance of the combustion zone. In 'all cases, -the initial air rate was in the order of a one cubic foot per minute and was gradually increased to the desired rate over several hours. This low initial air rate was necessary to avoid extinguishing combustion before a sufiicient preheating zone existed in the formation to preheat the relatively cold incoming air. Total duration of each combustion test averaged three days. Thermocouples were disposed at various points in the formation to obtain indications of the temperatures prevailing therein.

The rate of burning was observed to be a direct function of the rate of flow of the injected air. The average rate of advance of the combustion zone ranged from 0.25 feet per day for an air injection rate of 2 cubic feet per hour to a rate of advance of 8 feet per day, corresponding to an air injection rate of cubic feet per hour. The average temperature of the combustion zone was 1000" F., and the temperature ranged from 700 F. to 1800 F. Common characteristics of all of the tested formations were as follows: porosity, 60%; oil saturation, 50%; water saturation, 40%; gas saturation, 10%; oil gravity, 25 API; permeability, 6 darcies; thickness of the formation, 6 feet. The following table lists the results of a number of the tests on these blocks in terms of varying conditions.

Table 1 Variable Test I Test II Test III Test IV Test V Mass Velocity (pound moles of oxygen per hour per square foot of crosssectional area) 0.0032 0.0022 0.012 0.021 0.041 Burning Rate (inches per hour) .15 0.7 1.5 3

1 Thermally unstable combustion extinguished.

Sampling of the formations after combustion was extinguished clearly revealed the outlines of the burning patterns. The portions of the formations which had been passed over by the combustion zone were a brick-red color, and the combustion zones of the different blocks appeared as black zones, colored by deposits of coke.

Minor variations in the average permeability of the formations appeared to have no effect on the rate of advance of the combustion front, and burning proceeded relatively uniformly despite the presence of cracks or other areas of non-uniform permeability. This compensation of permeability variation is a result of continuous redistribution of fluids by heat from the combustion zone. It appears that at first, the crack or other portion of nonuniform permeability receives a disproportionately large amount of air, and therefore, for a time, burning proceeds more rapidly in this portion. However, this increase in burning rate produces an increased buildup of liquid saturation downstream of the crack to cause a reduction in air permeability of the crack, with a consequent diversion of air away from this portion. Therefore, the burning rate of this portion decreases, so that the average burning rate tends to be equal throughout the formation.

I claim:

1. The method of recovering gas and oil from ,a subterran'ean formation containing a petroliferous deposit and penetrated by at least one input well and at least one adjacent output well, comprising the steps of heating said formation adjacent said input well to a sufliciently elevated temperature to establish a combustion zone in said I v said formation adjacent said combustion zone at a temperature less than the temperature of said combustion zone, and maintaining the mass velocity of said free oxygen within the limits of 0,016 953 to 1.08 58 pound moles per hour per square foot of cross-sectional area of said combustion zone, where isthe porosity of said formation expressed as the decimal equivalent of. theratio of the actual formaion porosity to unit porosity, and Sis the oil saturation of said formation expressed as the decimal equivalent of the ratio of the actual oil. saturation in the formation totcom'plete saturation, while recovering-gasv and oil from said output well. a r

2. The method in accordance with claim l'wherein said free oxygen is continuously introduced. v :H;

3. The method in accordance with claim 1 in which said free oxygen is intermittently introduced.

- References Cited in the file of this patent UNITED STATES PATENTS 2,642,943 Smith et al. June 23, 1953 

